Infectious salmon anemia virus (ISAV) replication is transiently inhibited by Atlantic salmon type I interferon in cell culture.

Infectious salmon anemia virus (ISAV) is a piscine orthomyxovirus, which causes multisystemic disease in farmed Atlantic salmon that may result in large losses. Previous work has suggested that ISAV is able to resist the antiviral state induced in cells by type I interferon (IFN). These studies were, however, mainly based on cytopathic effect (CPE) reduction assays. Here we have investigated the antiviral activity of Atlantic salmon IFNa1, IFNb and IFNc against ISAV using quantitative PCR (qPCR) of segment 6, Western blot analysis of ISAV proteins and viral yield reduction assays, in addition to CPE reduction assays. Antiviral effects of IFNs were tested against the high virulent strain ISAV4 and the low virulent strain ISAV7 both at the optimum growth temperature 15°C and at 20°C. As expected, IFNa1 showed little protection against CPE development in cells after infection with both strains at 15°C. However, the qPCR and Western blot analysis clearly showed strong inhibition of replication of the virus strains by IFNa1 between 24 and 72h after infection. The inhibitory effect declined four to five days post-infection, which explains the low protection against CPE development 7-10 days later. At 20°C, IFNa1 showed strong protection against CPE development, probably due to slower virus growth. IFNc showed similar antiviral activity as IFNa1 against ISAV4 while IFNb showed lower activity. There were observed differences between ISAV4 and ISAV7 both with respect inhibition by IFNa1 and ability to induce the two IFN-inducible antiviral effector proteins, Mx and ISG15, which may be related to differences in virulence properties and/or adaption to growth in cell culture.